According to one of the conventional methods, in the current control, a deviation of output current from a calculated current command value (or vector value) is inputted to a comparator, and then an output of the comparator indicating a comparison result is used to determine the switching pattern of the PWM inverter, i.e. on/off of the branches of the inverter.
In other words, in the conventional method, there is only one state variable or state vector to determine the switching.
However, according to the method, the optimum switching is not always accomplished because the switching method is determined by only one state variable.
For example, some high frequency currents and acoustic noises have been observed. Moreover, there exists another problem in which the structure of the circuits for obtaining a command value are complicated because the one command value is used to control the whole system.
It is the object of the present invention to solve the above-mentioned problems.